Distortion
Distortion is an aberration occurring because an image is more or less magnified as the object is moves further off axis. Magnification of the image is dependent on the off-axis image distance.
Different areas of the lens have different focal lengths and different magnifications. This causes the image as a whole to be misshaped, even though each individual point is still sharply focused.
What does distortion look like?
Positive distortion is also called pincushion distortion. With positive distortion, each image point is displaced radially outward from the center, with the most distant points moving the greatest amount because the magnification increases with axial distance (figure 1b).
Negative distortion is also called
barrel distortion. With negative
distortion, the magnification decreases with axial distance, causing each point
on the image to move radially inward (figure 1c).
Smith,
Modern Optical Engineering, 2nd ed., 1990.
Hecht,
Optics, 4th ed., 2002. Hecht,
Optics, 4th ed., 2002.
Unless the aperture is flush with
the lens a stop in a system will always bring distortion. A stop alters which ray is the chief ray in
a system.
This means that if a stop is in front of the lens, the path length along the chief ray is longer, creating a smaller magnification. If a stop is placed behind the lens, the path length along the chief ray is decreased, increasing the magnification.
Hecht,
Optics, 4th ed., 2002.
Therefore,
introducing a stop between two lenses can make the resulting distortion
zero. The distortion is eliminated when
the pincushion distortion from the first lens and stop exactly cancels the
barrel distortion from the stop and second lens (figure 4).
Hecht, Eugene, Optics, 4th edition, San Francisco: Addison-Wesley, 2002.
Smith, Warren, Modern Optical Engineering, 2nd edition, New York: McGraw-Hill, Inc., 1990.